diff --git a/More/EAV/Attribute.php b/More/EAV/Attribute.php
index 46ba9f4..6c36cb2 100644
--- a/More/EAV/Attribute.php
+++ b/More/EAV/Attribute.php
@@ -15,7 +15,7 @@ class Attribute implements \Stringable
         $this->values = new SplObjectStorage();
     }
 
-    public function addValue(Value $value)
+    public function addValue(Value $value): void
     {
         $this->values->attach($value);
     }